The demand and growth of wireless communication services requires reliable and fast transmission of data and video with data rates of several Megabits per second. The fundamental phenomenon that makes reliable wireless transmission difficult is the time-varying multi-path fading, with the rates depending on the mobility of the user. Sending multiple copies of the same transmitted signal through possibly independent fading channels increases the probability that at least one of them will arrive at the receiver without being severely deteriorated. This technique is called diversity. It presents the single most important and well-established solution for the reliable wireless communication. Different diversity methods such as temporal, frequency, polarization or spatial diversity are successfully employed in the existing wireless communication system.
Wideband CDMA (code division multiple access) types of systems occupy a bandwidth typically several times larger than the channel's coherence bandwidth. Multi-path combining at the receiver turns the frequency diversity of the channel into an inherent benefit of the system. However, indoor wireless channels have large coherence bandwidths and, therefore, usually offer no frequency diversity. In order to circumvent this coherent bandwidth problem, it is possible to assign the same copy of transmitted signal to different uncorrelated transmit antennas and spreading each of the assigned copies with a different delayed version of the same spreading code, wherein each version is offset by a few chips. This method, known as the CDMA Delay Transmit Diversity scheme, creates artificial multi-path propagation and transforms a frequency non-selective channel into a frequency-selective channel.
Recent theoretical results in information theory have shown that multiple-input multiple-output (MIMO) wireless channels potentially offer a linear increase in link capacity, providing that antennas at the transmitter and receiver arrays are uncorrelated and that ubiquitous “key hold” effects do not occur in the channel. Multiple antenna systems with the corresponding signal design (coding and modulation) are, therefore, seen as a key solution for high demand on transmission speed and reliability in future wireless systems. When the channel state information (CSI) is not available at a transmitter, space-time coding (STC) is an optimal signaling strategy, designed to reach the theoretical limits on MIMO Rayleigh fading channel capacity by simultaneously coding across the spatial domain and the temporal domain. However, the complexity of the STC increases exponentially with the number of transmit antennas. In a theoretically optimal STC system, the complexity would reach the point when maximum likelihood decoding (MLD) becomes impractical or even unrealizable.
Lower complexity sub-optimal schemes based on the combining of classical single antenna channel coding with MIMO signal processing have recently gained a huge interest. Current 3GPP standardization for high speed down-link WCDMA FDD, as disclosed in “3rd Generation Partnership Project, Technical Specification Group Radio Access Network; Physical Layer Aspects of UTRA High Speed Downlink Packet Access” (3G TR25.848, v4.0.0 (2001-03)), is mainly concentrated around two proposals for multiple antenna transmission, VBLAST (Vertical Bell Labs Space-Time) and the tradeoff between rate, puncturing and orthogonality in space-time block codes for more than two transmit antennas.
VBLAST relies on spatial multiplexing at transmitter and spatial filtering at receiver, to enable employment of single antenna channel codes to MIMO systems. Detection is performed by successive nulling of layers, which are not yet detected, combined with decision-directed interference suppression of those layers previously detected. Spatial filtering at receiver requires the number of receive antennas to be greater than or equal to the number of transmit antennas, which might be impractical for down-link type of systems. Due to linear processing used to suppress interfering signals, dominant diversity in this architecture is one. Applying powerful channel coding with iterative turbo detection (inter-antenna interference suppression) and decoding was also considered as a way to improve the performance, but the drawback of such approach is a further dramatic increase in receiver complexity.
A generalization of VBLAST, as introduced in Tarokh et al. “Combined Array Processing and Space-Time Coding” (IEEE Trans Inf. Th. vol. 45, no. May 4, 1999), proposes the application of lower complexity two antenna space-time trellis codes to more than two transmit antennas. Antennas at transmitter are partitioned into pairs, and individual space-time trellis codes (STTC) (component codes) are used to transmit information from each pair of transmit antennas. More powerful space-time codes for two transmit antennas, i.e., space-time turbo coded modulation (STTuCM) can be readily applied as component codes. At the receiver, an individual space-time code is decoded with the help of a liner array processing (LAP) technique called, “the group interference suppression method”, that suppresses signals transmitted by other pairs of transmit antennas treating them as interference. The above method enables the number of receive antennas to be reduced by half as compared to VBLAST. Similar to VBLAST, the performance can be further improved by iterative, inter-antenna interference suppression and decoding with the prize of increased system complexity.
Single antenna channel codes and space-time codes applied to VBLAST and its generalization can be employed as a horizontally- or vertically-coded system with, the difference coming from position of the block for serial-to-parallel conversion before or after the encoding block, respectively. A horizontally-coded system will enable improved decoded-based, decision-directed interference suppression, and the vertically-coded system is expected to benefit from averaged SNR (signal-to-noise ratio) over successive layers, i.e., spatial interleaving.
Space-time block codes (STBC) were originally introduced as a simple transmit-diversity scheme (STTD) for power efficiency improvement by employing two antennas at the transmitter. STTD was later generalized to an arbitrary number of transmit antennas, though the schemes for more than two transmit antennas have a drawback of decreased rate as compared to single transmit antenna systems. The main benefit of space-time block codes is a simple yet efficient exploitation of transmit antenna diversity, but even if some optimality is compromised for retrieved rate, the overall throughput is no higher than in single transmit antenna systems.
The spatial multiplexing of signals over different transmit antennas employed in VBLAST and its generalization assumed separation of transmitted signals only at receiver. Being spreaded by the same spreading code and simultaneously transmitted over n different transmit antennas, signals arriving at the given receive antenna destructively interfere each other. To detect the signal coming from the first transmit antenna, i.e., the first layer, n−1 interfering signals are nulled out by a linear ZF (zero-forcing) or MMSE (minimum mean square error) based spatial filtering method that requires a minimum of n antennas at the receiver. After the first layer has been detected, its contribution to the received signals on different receive antennas is subtracted and detection of the next layer is performed in the same fashion. The above method increases the complexity of the mobile handset in the downlink and has obvious limitations in the throughput determined by the minimum required number of antennas in the receiver. Due to the linear processing at the receiver, the dominant diversity in the system is one.
It is advantageous and desirable to provide a method and system for spatial multiplexing of signals over different transmit antennas wherein the complexity can be reduced so that the method and system can be used in a mobile handset.